In recently years, improvement in the fuel economy of automobiles has recently been required to regulate the amount of CO2 emission from the viewpoint of global environment conservation. In addition, it is also required to improve safety by focusing on collision characteristics of automobile bodies in order to ensure the safety of passengers in case of accident. Thus, a reduction in the weight of automobile bodies and an increase in the strength of automobile bodies are being actively promoted.
To achieve both of the reduction in the weight and the increase in the strengthening of automobile bodies, an increase in the strength of a material for members to the extent that rigidity is not impaired and a reduction in weight by reducing the thickness of sheets are said to be effective. Nowadays, high-strength steel sheets are positively used for automotive parts. Steel sheets used for structural members and reinforcing members for automobiles each have a tensile strength (TS) of 780 MPa or more and even 980 MPa or more. The transformation strengthening is effective in increasing the strength of steel sheets. Multiple phase steel sheets composed of soft ferrite and hard martensite have good ductility and a good strength-ductility balance and thus have relatively good press formability. However, in multiple phase steel sheets, changes in conditions, such as annealing temperatures, in production in common continuous annealing lines cause significant variations in material properties, such as tensile strength (TS), so that the material properties are liable to vary in the longitudinal direction of coils. In this case, it is difficult to stably performing press forming in continuous press lines, thereby seemingly causing a significant reduction in workability. To ensure the amount of martensite required for strengthening, the amounts of C, Mn, and so forth are increased in multiple phase steel sheets. Mn is a more oxidizable element than Fe. Thus, in the production of galvanized steel sheets and galvannealed steel sheets with high Mn contents, ensuring wettability is problematic. Specifically, Mn in steel is selectively oxidized even in a non-oxidizing atmosphere or reducing atmosphere used in a common annealing furnace, concentrated on a surface to form an oxide, thereby reducing wettability with molten zinc at the time of coating treatment and possibly causing an uncoated portion.
In contrast, Patent Literature 1 discloses a method for improving wettability with molten zinc, the method including heating a steel sheet in an oxidizing atmosphere in advance to rapidly form an Fe oxide film on a surface at an oxidation rate of a predetermined rate or more and to inhibit the oxidation of added elements on a surface of a steel sheet, and then performing the reduction annealing of the Fe oxide film. However, in the case of a large amount of the steel sheet oxidized, iron oxide can adhere to a hearth roll to disadvantageously cause pressing flaws on the steel sheet. Patent Literature 2 discloses a method in which after annealing, a steel sheet is subjected to pickling to remove an oxide on its surfaces, annealing again, and galvanizing. However, Patent Literature 2 states a 590 MPa grade steel sheet, but contains no description of a steel sheet having a tensile strength (TS) of 780 MPa or more. Furthermore, Patent Literature 2 contains no description of elongation properties and variations in material properties serving as indices of press formability.